A three-way catalyst (TWC) has been used to purify exhaust gas exhausted from a gasoline-powered vehicle and including HC, CO, and NOx. The TWC serving as an exhaust gas purification catalyst is usually designed such that the TWC exhibits high activity when the concentrations of HC, CO, NOx, and the like and the concentration of oxygen are in specific ranges (also referred to as a window). Buffering of change in the oxygen concentration exhibits action for retaining such a window range and is useful for removing hazardous components in the exhaust gas with high efficiency.
These HC and CO in the exhaust gas are oxidized by platinum group metals such as platinum (Pt), palladium (Pd), and rhodium (Rh). The platinum group metal serving as the catalyst component is supported on a heat resistant inorganic oxide having a high surface area such as activated alumina in a highly dispersed manner and coats a honeycomb structure type carrier in the form of catalyst composition slurry together with other catalyst materials (refer to Patent Literature 1).
Pt, which is an oxidizing active species, has particularly high activity and still exhibits high purification performance even with poisoning of Pt or particle growth. Therefore, Pt has been widely used as a catalyst for purifying exhaust gases exhausted from internal combustion engines of automobiles or the like. Reduction in the amount of Pt used, however, has been studied from the viewpoint of resource protection and cost.
As a solution for reducing the amount of Pt used, replacing at least a part of Pt with Pd has been studied. Although both Pt and Pd are active species having an oxidizing function, Pd causes significant deterioration in the activity due to poisoning by sulfur and the like or particle growth. Pd is more easily alloyed when used in combination with Rh than when Pt is used. Under severe conditions such as high temperature oxidizing atmosphere, particles of Pd are grown and performance may deteriorate due to unfavorable interaction with co-catalyst components or poisoning components in exhaust gas. Therefore, Pd is used with a component suppressing to poisoning, sintering, particle growth, and alloying (refer to Patent Literature 2 and Patent Literature 3).
The exhaust gas from automobiles contains various reactive components and has high temperature, and thus the exhaust gas purification catalyst components are easily sintered to cause poisoning. In the exhaust gas purification catalyst, the major part of its purification active species is a noble metal and thus reduction in poisoning of the noble metal and suppression of sintering are important problems. Various solutions have been studied for these problems.
NOx such as NO and NO2 in exhaust gas are air pollutants and N2O is a greenhouse gas promoting global warming. Therefore, government organizations in various countries execute various regulations on emission of NOx.
Rh is used as the catalytically active species for the removal of NOx. Rh, however, is a material that may cause alloying when Rh is used with Pd in the same composition (refer to Patent Literature 2). When Rh is used together with Pt and Pd, which are oxidizing active species, in the same catalyst composition, the oxidation performance and the reduction performance may be canceled out. Therefore, coating a honeycomb structure type carrier with Rh, Pt, and Pd individually as different catalyst compositions has been studied.
In addition to the active species as described above, co-catalyst components selected from an oxygen storage and release material (hereinafter may be referred to as OSC (oxygen storage component)), a barium (Ba) component, and inorganic oxides such as zirconia, silica, titania, alumina, and zeolite are frequently employed as the exhaust gas purification catalyst. This OSC stores oxygen when the oxygen concentration in the exhaust gas is high and releases oxygen when the oxygen concentration in the exhaust gas is low. A change in the oxygen concentration in the exhaust gas is buffered by the storage and release of oxygen and the oxygen concentration (window range) suitable for purification of the exhaust gas can be controlled.
When the oxygen concentration in the exhaust gas is low, the oxidation of HC and CO is difficult to promote. In such a case, OSC supplies oxygen into the exhaust gas, oxidizes HC and CO, and functions to promote exhaust gas purification. Such an action may be referred to as an oxidation-reduction reaction. When OSC having high oxygen supply and storage speed is used, a catalyst having excellent HC and CO removal capability tends to be obtained. Cerium-zirconium composite oxide is known as OSC having high oxygen storage and release speed (refer to Patent Literature 4).
The conceivable reason why the storage and release speed of oxygen is fast is that the cerium-zirconium composite oxide has a stable crystal structure in both heat and oxidation reduction, does not inhibit the function of the cerium oxide serving as the main component of OSC, and can be used for functioning as OSC to the inside of the particles.
In the exhaust gas purification catalyst, the Ba component is usually used as a co-catalyst component. The Ba component has a function of adsorbing NOx in the exhaust gas. More specifically, in the case that the Ba component is BaCO3, BaCO3 reacts with NOx to be Ba(NO3)2 when the NOx concentration in the exhaust gas increases. Such a reaction with NOx may be referred to as adsorption of NOx or storage of NOx.
Generally, NOx is generated in a large amount when the amount of fuel supplied to an engine is relatively smaller than the amount of air. The Ba component temporarily stores NOx thus generated. When NOx is stored by the Ba component, the concentration of NOx in the exhaust gas decreases, and when the CO concentration increases, NOx is released from the Ba component. This is because Ba(NO3)2 reacts with CO to be BaCO3. NOx released from the Ba component reacts with reducing components in the exhaust gas on the surface of the active component such as Rh and is reduced and removed. Such storage and release of NOx with the Ba component are referred to as storage and release due to the chemical equilibrium of the Ba component.
In addition to such OSC and Ba components, zirconia is frequently used as a co-catalyst component. Zirconium is a transition metal and zirconia, which is an oxide of zirconium, also has oxygen storage and release capability. Therefore, zirconium oxide may be used as OSC. Zirconium oxide, however, is believed to have not so high capability as OSC, as compared with cerium oxide.
Zirconia rather improves the NOx removal performance by promoting steam reforming reaction. Consequently, it has been conceivable that steam reforming reaction is promoted as follows (refer to Patent Literature 5) by using zirconia together with the Rh component in TWC.HC+H2O→COx+H2  (1)H2+NOx→N2+H2O  (2)
It has been known that such a steam reforming reaction proceeds in a relatively high temperature atmosphere. A catalyst having a high NOx removal capability even at low temperature, however, is desired.
For the removal of NOx in the exhaust gas, a method of directly utilizing reducing components in the exhaust gas is included, in addition to a method for utilizing hydrogen generated by the above steam reforming reaction. One of these reducing components includes carbon monoxide (CO). The removal of NOx using CO is referred to as CO—NO reaction and it has been known that this reaction proceeds even at relatively low temperature (for example, Patent Literature 6). However, no specific means that satisfies the market requirements for the removal efficiency of NOx by the CO—NO reaction has been developed.
Under such circumstances, the present applicant has developed a honeycomb structure type catalyst in which a carrier is coated with two or more layers of catalyst compositions to remove carbon monoxide, hydrocarbons, and nitrogen oxides contained in exhaust gas, the upper layer side catalyst layer contains palladium supported on a heat resistant inorganic oxide, an oxygen storage and release material, and a barium component, and the lower layer side catalyst layer contains rhodium supported on a cerium-zirconium composite oxide having an cerium-zirconium weight ratio in terms of oxides of 0.05 to 0.2 (refer to Patent Literature 7).
Such development enabled the present applicant to provide a catalyst that exhibits excellent exhaust gas purification performance capable of meeting the various regulations, is inexpensive, and has less deterioration in the purification performance even after long-term use. However, the removal rates of carbon monoxide (CO) and nitrogen oxide (NOx) have been particularly low during a period when the temperature of the catalyst bed soon after the engine start-up is relatively low.
Therefore, in order to obtain a catalyst having high NOx removal capability even at low temperature, a catalyst having a high [CO+NO] reaction capability, that is, a three-way catalyst having high removal rates of carbon monoxide (CO) and nitrogen oxides (NOx) even during the period when the temperature of the catalyst bed is relatively low and having relatively low cost has been required.